1. Field of the Invention
The present invention relates to leak detecting circuits for power source devices for use in electric motor vehicles and the like, and more particularly to a leak detecting circuit for a power source device having a cell unit comprising a plurality of cells.
2. Description of the Related Art
Electric motor vehicles such as hybrid cars are provided with a cell unit serving as a power source for the drive motor and comprising a plurality of secondary cells connected in series. Such a cell unit produces a high voltage, for example, of at least 240 V, so that if a leak occurs, a leakage current flowing portion is likely to provide an electric shock to the person by contact, posing the problem of bringing a danger to the human body. Accordingly, it is conventional practice to provide a circuit for detecting leaks, as shown in FIG. 5, to notify a control system or the driver of the occurrence of a leak upon detecting the fault. The leak detecting circuit of FIG. 5 has a cell unit Vb which comprises about 200 nickel-hydrogen cells NiMH connected in series, and an output voltage, which is usually about 240 V, is available across a positive (P) terminal and a negative (N) terminal. Voltage dividing resistors R1 and R2 (R1=R2) of high resistance value are connected between the P and N terminals, and the midpoint between the two resistors is grounded via a resistor R3. The voltage between the opposite ends of the resistor R3 is detected by a voltmeter V. In the case of electric motor vehicles, xe2x80x9cgroundingxe2x80x9d means connection to the chassis (vehicle body).
When no leak occurs, no current flows through the resistor R3, with the result that the voltmeter V delivers zero output. If a leak occurs in the line P, the line P is grounded via a ground-fault resistance RL in the resulting equivalent circuit as indicated in a broken line in the diagram. Formed in the equivalent circuit is a current path of line Pxe2x86x92 resistance RLxe2x86x92 resistor R3xe2x86x92 resistor R2xe2x86x92 line N, and a voltage is produced between the opposite ends of the resistor R3. The leak can be detected by detecting the voltage thus produced by the voltmeter V. The voltmeter V has the function of measuring the voltage in detecting the leak and judging whether the measurement is not lower than a predetermined value or below the predetermined value, and can therefore be provided by a voltage comparator.
In FIG. 5, the resistors R1, R2, R3 provide insulation resistance for the cell unit against ground, and the combined resistance value of these resistors is substantially an insulation resistance value. Usually the circuit is so desired that the combined resistance value will not be less than 1 Mxcexa9. Assuming that the input impedance of the voltmeter V is Zv, the insulation resistance value Rz in FIG. 5 is calculated from Mathematical Expression 1.
Rz=R1xc2x7R2/(R1+R2)+R3xc2x7Zv/(R3+Zv)xe2x80x83xe2x80x83(Mathematical Expression 1)
When the resistance values of the resistors R1, R2 are small, the cell voltage Vb remains applied to the resistors R1 and R2 to discharge the cell unit at all times, so that the resistors R1, R2 have a relatively high resistance value. For example, in the case where the cell voltage is 240 V and the limit value for the allowable discharge current is 100 xcexcA, the combined resistance value of the resistors R1, R2 is at least 2.4 Mxcexa9. Accordingly, the resistors R1, R2 have a high resistance value with reference to this resistance value. As a result, the insulation resistance value Rz of Mathematical Expression 1 is such that the first term becomes predominant which makes it possible to provide a sufficiently great value to the insulation resistance value Rz even if the resistance R3 and impedance Zv of the second term have relatively small values. Usually, the resistor R3 is designed to have a small resistance value so that the input impedance Zv of the voltmeter V is negligible.
FIG. 6 shows the construction of another conventional leak detecting circuit. The circuit has voltage dividing resistors R1 and R2 of high resistance value connected between P and N terminals. The midpoint between the two resistors is grounded. The potential difference between the line P and the grounded point is detected by a voltmeter V1, and the potential difference between the line N and the grounded point by a voltmeter V2. To afford a sufficiently great insulation resistance, the resistors R1, R2 have a high resistance value, for example, of at least 2.2 Mxcexa9.
In the leak detecting circuit of FIG. 6, a high voltage of Mathematical Expression 2 below is steadily applied to the voltmeters V1, V2 even in the event of no leak occurring.
V1=V2=(1/2)Vbxe2x80x83xe2x80x83(Mathematical Expression 2)
If a leak occurs in the line P, the line P is grounded via a ground-fault resistance RL in the resulting equivalent circuit as indicated in a broken line in the diagram. Accordingly, the leak is detectable from a decrease in the voltage detected by the voltmeter V1 or an increase in the voltage detected by the voltmeter V2.
Power source devices comprising a cell unit include those which require detection of the xe2x80x9cdegreexe2x80x9d of leaks. In the event of a leak occurring, for example, there arises a need to judge whether a countermeasure should be taken urgently or in due time in accordance with the xe2x80x9cdegreexe2x80x9d of the leak. Stated more specifically, a judgement is made in accordance with the magnitude range of ground-fault resistance RL as shown in FIG. 7. The value of ground-fault resistance RL, when not smaller than a threshold value RL2, indicates occurrence of no leak, hence safety. If the value RL is within the range of a threshold value RL1 to the value RL2, this indicates the occurrence of a leak, which nevertheless is not dangerous to the human body. When up to the threshold value RL1, the value RL is interpreted as indicating a danger to the human body. For example, if resistance values dangerous to the human body are up to about 100 xcexa9/V (at least 10 mA calculated as current), the threshold value RL1 is 24 kxcexa9 when the output voltage is 240 V. However, if the threshold values RL1, RL2 are great, an increased impedance will result, causing the circuit to make an error in detection, so that the threshold values RL1, RL2 are determined in the range of 50 to 70 kxcexa9, with an allowance made for the value 24 kxcexa9.
FIG. 8 shows the leak detecting circuit of FIG. 5 when R1=R2=2.2 Mxcexa9, R3=10 kxcexa9 (Zv is sufficiently great relative to R3), and Vb=240 V. In the case where the ground-fault resistance RL has varying values of 0 xcexa9, 50 kxcexa9, 70 kxcexa9, 1 Mxcexa9, 2 Mxcexa9 and ∞, the voltage values to be detected by the voltmeter V are listed in Table 1.
With reference to FIG. 8, two voltmeters VA, VB connected in parallel with the resistor R3 are for use in detecting leaks when the ground-fault resistance is up to 70 kxcexa9 and up to 50 kxcexa9, respectively. The voltmeter VA is adapted to detect leaks at not lower than 1.017 V, and the voltmeter VB operates for detection at not lower than 1.034 V as will be described below more specifically.
(1) Neither of the voltmeters VA, VB operate for detection at not higher than 1.017 V. This case is interpreted as indicating occurrence of no leak.
(2) The voltmeter VA operates for detection when the voltage to be detected is in the range of 1.017 to 1.034. The voltage is interpreted as indicating the occurrence of a leak in the precaution range shown in FIG. 7.
(3) Both the voltmeters VA, VB operate for detection when the voltage to be detected is at least 1.034 V. The voltage is interpreted as indicating the occurrence of a leak in the dangerous range shown in FIG. 7.
In the case where a leak occurs in the line N, the sign of the calculated values in Table 1 is reversed, so that the voltage detecting circuit is constructed so as to detect positive voltages as well as negative voltages, and the sign is taken into consideration in making a judgement according to the above criteria (1) to (3).
Table 1 shows that as the ground-fault resistance RL decreases from infinity (no leak), the voltage to be detected increases. While the maximum variation in the voltage to be detected is 1.081 V, the difference between 50 kxcexa9 and 70 kxcexa9 in detected voltage is as small as 17 mV (1.6% of the maximum variation 1.081 V). Accordingly, there is a need to use voltmeters of very high precision for accurate judgement.
With the leak detecting circuit of FIG. 6, the precision required of the voltmeters can be lower. FIG. 9 shows a specific design example, and Table 2 shows variations in the voltage to be detected at varying ground-fault resistance values.
As shown in Table 2, the difference between the ground-fault resistances 50 kxcexa9 and 70 kxcexa9 in the voltage to be detected is increased to 1.962 V. In the event of no leak occurring, 120 V is applied to the voltmeters, and the accuracy achieved at this voltage is about 1.6%. With the leak detecting circuit of FIG. 6, however, it is difficult to maintain a high insulation resistance because the cell unit is grounded via the voltmeters. Further the circuit involves the problem that a high voltage is applied to the pair of voltmeters in the event of a leak occurring.
The leak detecting circuit of FIG. 6 is modified into the circuit of FIG. 10 which is adapted to detect leaks individually when the ground-fault resistance is 50 kxcexa9 and 70 kxcexa9. With reference to FIG. 10, voltmeters V1A, V2A are adapted to detect leaks due to a ground-fault resistance of 70 kxcexa9, and voltmeters V1B, V2B are for use in detecting leaks due to a ground-fault resistance of 50 kxcexa9.
Any of the circuits shown in FIGS. 8, 9 and 10 detects the occurrence of leaks in terms of the absolute values of voltage levels, so that the leak voltage value is obtained merely as a xe2x80x9cvalue at a particular cell voltage.xe2x80x9d However, the actual cell voltage varies over a wide range, for example, from 280 V at the fully charged state to 120 V at the final stage of discharge. The calculated values in Table 1 or 2 also vary with these variations in cell voltage, such that even at a particular leak resistance value, the voltage can be detected as indicating a leak or will not be detected as indicating a leak, depending on the charged state. More specifically, the voltage to be detected increases with an increase in cell voltage, so that a problem is encountered in that a leak will be detected even if the leak resistance value is so great that there is essentially no need to detect any leak.
An object of the present invention is to provide a leak detecting circuit for use in power source devices which is capable of accurately detecting a leak which occurs due to a relatively low ground-fault resistance, for example, of tens of kxcexa9. Another object of the invention is to provide a leak detecting circuit which is adapted to detect a leak due to a predetermined ground-fault resistance by detecting a specified voltage even when the output voltage of the power source device varies.
For use in power source devices, the present invention provides a leak detecting circuit wherein a pair of positive and negative power lines extending from respective opposite electrodes of a cell unit are connected to each other by four voltage dividing resistors R1A, R1B, R2A, R2B connected to one another in series, and a midpoint between the two voltage dividing resistors R1A, R1B arranged on the positive power line side and the two voltage dividing resistors R2A, R2B arranged on the negative power line side is grounded. The potential difference between the positive power line and the midpoint between the two voltage dividing resistors R1B, R1B on the positive side and the potential difference between the negative power line and the midpoint between the two voltage dividing resistors R2A, R2B on the negative side are compared with respective reference voltages to detect a leak.
The reference voltage on the positive side is set at a potential difference to be produced between the positive power line and the midpoint between the two voltage dividing resistors R1A, R1B on the positive side when a leak occurs due to a ground-fault resistance value to be detected, and the reference voltage on the negative side is set at a potential difference to be produced between the negative power line and the midpoint between the two voltage dividing resistors R2A, R2B on the negative side when a leak occurs due to a ground-fault resistance value to be detected.
In the leak detecting circuit of the invention described, the voltage obtained between opposite electrodes of the cell unit is divided by the four voltage dividing resistors R1A, R1B, R2A, R2B. The voltages consequently produced across opposite ends of the respective resistors R1A, R2A which are positioned at the respective ends are compared with respective reference voltages. In the absence of a leak, the voltage is greater than the reference voltage, indicating that no leak is occurring.
On the other hand, suppose a leak occurs, for example, in the positive power line. An equivalent circuit is then formed wherein the positive power line and the midpoint of the four voltage dividing resistors are connected to each other via a ground-fault resistance. As a result, the potential at the midpoint between the two voltage dividing resistors R1A and R1B on the positive side increases, consequently lowering the voltage across the resistor R1A to below the reference voltage. This indicates the occurrence of the leak. If a leak occurs in the negative power line, the voltage across the resistor R2A similarly decreases to below the reference voltage, indicating the occurrence of the leak.
Specifically stated, the pair of positive and negative power lines extending from the respective opposite electrodes of the cell unit are connected to each other by at least three auxiliary voltage dividing resistors R3, R4, R5 connected to one another in series, and a plurality of potential reference points are provided between the auxiliary voltage dividing resistors. A positive-side voltage comparison circuit V1 is interposed between one of the potential reference points and the midpoint between the two voltage dividing resistors R1A, R1B on the positive side, and a negative-side voltage comparison circuit V2 is interposed between the another one of the potential reference points and the midpoint between the two voltage dividing resistors R2A, R2B on the negative side.
In this specific arrangement, the voltage produced at the potential reference points between the three auxiliary voltage dividing resistors R3, R4, R5 varies in proportion to the variations in the voltage output from the cell unit. Using the voltage as the reference voltage for the detection of leaks, the positive-side voltage comparison circuit V1 compares the voltage of the midpoint between the positive-side two resistors R1A, R1B with the reference voltage, and the negative-side voltage comparison circuit V2 compares the voltage of the midpoint between the negative-side two resistors R2A, R2B with the reference voltage. This ensures the detection of leak free of the influence of variations in the output voltage of the cell unit.
Further stated specifically, a positive-side voltage limiter is interposed between the positive power line and a positive terminal of the positive-side voltage comparison circuit V1 for limiting the potential difference between opposite ends of the voltage limiter to not higher than a predetermined value, and a negative-side voltage limiter is interposed between the negative power line and a negative terminal of the negative-side voltage comparison circuit V2 for limiting the potential difference between opposite ends of the negative-side voltage limiter to not higher than a predetermined value.
With this specific construction, the potential difference between the positive power line and the positive terminal of the positive-side voltage comparison circuit V1 is limited to not higher than a predetermined value, and the potential difference between the negative power line and the negative terminal of the negative-side voltage comparison circuit V2 is limited to not higher than a predetermined value. Accordingly, the positive-side comparison circuit V1 and the negative-side comparison circuit V2 to be used can be a circuit having a sensitivity region only in the voltage range of up to the predetermined value. This makes it possible for the comparison circuits to achieve improved voltage detection accuracy.
Thus, the leak detecting circuit of the invention for power source devices can detect leaks due to relatively low ground-fault resistances of tens of kxcexa9 with high accuracy. Further even if the output voltage of the power source device varies, leaks due to a predetermined ground-fault resistance can be detected at a specified detection voltage.